Research On The Microstructure And Wear Properties Of Plasma Transferred Arc (PTA) Modifying On Nodular Cast Iron

The surface of a component is generally one of the most important engineeringfactors. While it is in use it is often the surface of a work-piece that suffers from wear.In industrialized countries some30%of all energy generated is ultimately lost throughfriction. In the highly industrialized countries losses due to friction and wear are put atbetween1%and2%of Gross Domestic Product (GDP). Therefore, the complication ofthe tribological properties of materials and the economic respects of wear justifyincreasing research efforts. The development of novel wear-resistance materials hasbecome a key factor in modern industry and also become research hotspots all over theworld. Currently, the fabrication of wear resistant layers on metallic materials throughhigh energy sources (such as plasma beam, laser and electron beam) modifying thephysical and chemical properties of surface has become an effective and economic wayto improve the quality of products, to save precious and strategic materials, to developthe technology of maintenance and reproduction and extend the service life time ofmechanical products. In this present study, a plasma transferred arc (PTA) was used tosurface remelting and alloying a high vanadium high-speed steel coating on aferrite-matrix nodular cast iron (NCI). The optical microcopy (OM), environmental/field scanning electron microscopy (E/FSEM), X-ray fluorescence (XRF), X-raydiffraction (XRD), microhardness tester and dry sliding testing machine were used totest and systematically analyze the microstructure and mechanical properties of PTAhardfacing layers.Firstly, the PTA-remelted layer is divided into three different areas: remelted zone(RZ), heated affected zone (HAZ) and the substrate (SUB). Microstructuralcharacterization indicates that the PTA remelting treatment could dissolve most graphitenodules and that cementite, martensite, an inter-dendritic network of eutectic ledeburite,refined pearlite and certain residual austenite were transformed from the crystallized primary austenite dendrites during the rapid solidification. This structure has typicalcharacteristics of white iron. The introduction of the double shell structure consisting ofaphanite martensite and modified ledeburite effectively enhances the hardness of theinterface of the HAZ and substrate. The dimensions of the remelted zone and itsdendrites increase with increased energy density. And structure becomes coarsenedunder too slow scanning speed and too high current arc. The best parameter of PTAremelting is at the scanning speed of500mm/min with the current arc of90A. Themicrohardness of the remelted zone varied in the range of630HV0.2to818HV0.2,which is approximately3.15-4.09times as the hardness of the substrate.Secondly, a coating with similar composition of high vanadium high-speed steel(HVHSS) was deposited evenly on a ferrite-matrix ductile iron and hard-faced byplasma transferred arc (PTA). The PTA-alloyed surface has obvious metallic colour andis smooth and flat, without obvious cracks or porosities. A structure with a similar highvanadium high-speed steel-white cast iron-nodular cast iron from outside to inside ofthe surface of nodular cast iron was fabricated. Materials characterizations show that thecross-section consists of four regions: alloyed zone (AZ), remelted zone (RZ), heataffected zone (HAZ) and unaffected substrate. The maximum microhardness emergingat the subsurface of PTA-alloyed layer is956.5HV0.2, which is4.78times as thesubstrate. The improvement is attributed to the formation of mixed hard-phases such asMC-type carbides, M7C3, Cr23C6, and martensite during nonequilibrium solidification,and the solution strengthening caused by the alloying elements as well as grain refiningthrough the rapid cooling. The shape of the MC-type carbides is nodular, granulous, androd, indicating that PTA alloying HVHSS can evidently improve the hardness and wearproperties of the nodular cast iron without compromising the substrate’s toughness.Thirdly, dry sliding wear tests at room temperature suggested that both the PTAremelting and PTA alloying technology could effectively enhance the wear properties ofthe surface of the NCI. The dominant wear mechanism of PTA-alloyed sample isadhesive plastic deformation. This result could be interpreted as the formation of thein-situ synthesized MC, M7C3, M23C6, alloyed cementite, martensite, and fine ledeburite,together with grain refining caused by self-quenching, which makes the hardened layers very difficult to be plastically deformed or plowed during the sliding process. Thewear-resistant ranking of the materials related is PTA alloying> PTA remelting> Mn13steel> nodular cast iron. The relative wear resistance of PTA HVHSS-alloyed layerand PTA remelted-layer is22.55and10.76times as the substrate, respectively.